1. Technical Field
This invention generally relates to refrigeration apparatus and in particular relates a method and apparatus for detecting and preventing compressor slugging in a refrigeration compressor.
2. Background Art
Introduction of a hydraulic fluid into an operating refrigeration compressor i.e. "slugging," causes severe damage to the compressor and further results in an inoperative cooler. In the case of food storage coolers, this can result in the loss of thousands of dollars worth of product.
Compressor slugging is normally the result of an over designed cooling system wherein there is insufficient heat to insure complete evaporation of the low pressure liquid to a low pressure gas. If the low pressure liquid enters the refrigeration compressor, the compressor will go hydraulic and is incapable of compressing the fluid. The result is likened to that of the freezing of a piston in an internal combustion engine.
PARKER, U.S. Pat. No. 3,766,747, teaches a method and means for preventing operation of reciprocating refrigerant compressor in the event of the presence of a liquid slug. PARKER uses a state of matter sensor adjacent the entrance to the inlet valve designed to sense the presence of a predetermined quantity of liquid and prevent the operation of the compressor in the presence thereof. PARKER specifically teaches the use of a thermistor as the state of matter sensor which is capable of detecting only temperature differences. Because of the thermal dynamics involved in the workings of a refrigeration compressor, temperature sensing is not adequate as there is not always a sufficient temperature deferential between a low pressure gas and a low pressure liquid in a working refrigeration system.
It has been found through experimentation that a slugging compressor emits unique vibrations and sounds in a specific frequency band of detectable energies. FIG. 1(a) shows the vibration wave form of a normally operating compressor over the frequencies to 0 to approximately 400 hertz. FIG. 16 shows the vibration wave form of a slugging compressor over the same frequency range. Simple comparison of FIGS. 1(a) and (b) shows that over the frequency band of 80 to 305 hertz, there is a detectable and significant increase in the energy of the wave form.
A vibration detection apparatus is taught by A. E. TILLEY ET AL., U.S. Pat. No. 3,269,320, which teaches the use of a geophone to detect the characteristic pounding of a partially dry oil well and enacts subsequent shutdown shortly after detection. The device as taught by TILLEY, is not frequency discriminating and hence when applied to refrigeration compressors would detect all vibrations, not just those of a slugging compressor.
What is needed is a vibration detection apparatus which is capable of detecting a slugging compressor and further of preventing operation of the compressor in the event of a slug detection.
It is therefore an object of this invention to provide an apparatus which is capable of detecting a liquid slug in a refrigeration compressor operable over the entire compressor cycle and is further capable of preventing operation of the compressor in the event of a liquid slug detection.